Use a preprocessor flag to optionally depend on dynamic linking header

Summary:
we need to install libdl in order to
```
#include <dlfcn.h>
```

I'm turning this feature on based on host os. If user wants to enable it, they need to make sure `dlfcn.h` header is available which it should for most UNIX-like host OSes. Added comments for that.

Reviewed By: digantdesai, dbort

Differential Revision: D47886884

fbshipit-source-id: fdf4db841bd066dcf47f16ea622e56e35f48e660

Author: larryliu0820

runtime/platform/system.h

@@ -14,7 +14,11 @@
  */
 #pragma once
 
-#if defined(__linux__) || defined(__APPLE__)
+/**
+ * To enable dynamic linking debugging capability on UNIX-like OS. If enabled
+ * and see an error like: `undefined symbol: dladdr`, install `libdl` to fix.
+ */
+#if defined(ET_USE_LIBDL)
 #include <dlfcn.h>
 #endif
 
@@ -30,7 +34,7 @@ extern "C" {
  * @retval The path to the shared library containing the symbol.
  */
 inline const char* et_pal_get_shared_library_name(const void* addr) {
-#if defined(__linux__) || defined(__APPLE__)
+#if defined(ET_USE_LIBDL)
   Dl_info info;
   if (dladdr(addr, &info) && info.dli_fname) {
     return info.dli_fname;

runtime/platform/targets.bzl

@@ -92,6 +92,13 @@ def define_common_targets():
         exported_deps = [
             ":compiler",
         ],
+        exported_preprocessor_flags = select(
+            {
+                "DEFAULT": [],
+                "ovr_config//os:linux": ["-DET_USE_LIBDL"],
+                "ovr_config//os:macos": ["-DET_USE_LIBDL"],
+            },
+        ),
         visibility = [
             "//executorch/...",
             "@EXECUTORCH_CLIENTS",